Typically in the production of building cladding sheets and other sheet metal products, pre-painted steel strip can be produced in a steel finishing mill. In such painting processes, paint is applied to a hot substrate strip as a liquid melted from a solid body of substantially solvent free paint composition by the contact of the body with, or the near approach of the body to the hot strip. In this context, the term "liquid" includes high viscosity liquids whose form may approach that of a soft plastic solid as well as easily flowing liquids.
The above described mode of applying liquid material to a hot substrate is referred to as "melt deposition" and the deposited liquid is commonly called and will hereinafter be referred to as the melt deposit.
Previously, the determination of the deposition rate of melt deposits for purposes other than painting has been attempted by controlling the contact pressure between the solid body and the substrate strip while maintaining constant all of the many other parameters effecting the deposition rate. Such a process is described in U.S. Pat. No. 3,630,802 to Dettling.
A problem when using Dettling type pressure controlled melt deposition processes is accurately controlling all of the parameters effecting the deposition rate thus making it difficult to obtain the low and constant deposition rates needed to produce thin paint coats of uniform thickness. This problem has led to the replacement of such processes in practice by the melt deposition technique described in Australian Patent No. 667716.
Briefly stated, Australian Patent No. 667716 discloses depositing a polymer based coating composition onto a side of a substrate metal strip moving at a constant speed by heating the strip to a temperature above the glass transition temperature of the composition and driving a solid block of the Composition towards the strip at a predetermined block speed. Apart from the block speed, the other operating parameters are only required to lie within a broad range of working values. Thus to apply a melt deposit to the strip at a precisely controlled deposition rate, it is only necessary to control the block speed without the need to closely control other operating parameters.
It is also disclosed in Australian Patent No. 667716 that the melt deposit which for thin paint coats is discontinuous is then spread over the surface of the strip by a pressure roll and emerges therefrom as a smooth, wet coating on the strip. A bead of liquid coating builds up on the strip on the upstream side of the pressure roll and the block speed may be adjusted in response to the bead size. The emergent strip then travels through a curing furnace, if necessary and is caused or allowed to cool to complete the process.